Effects Of Annealing Processes And Second-phase On Texture Of SFH IF Steel

In recent years, with the progress of metallurgical technology and thedevelopment of the automobile industry, oil consumption, safety and environmentalprotection problems is produced with the increasing of automobile production andauto possession. It is particularly important to produce high strength, lightweightautomotive steel sheet. As the third generation of automotive deep drawing steel, IFsteel has been widely applied to automobile production, due to its good deep drawingperformance.The annealing experiments of fine grain high-strength IF steels were carried out indifferent annealing process. The microstructure observation was studied by opticalmicroscopy, second particles morphology and composition analysis was carried outon the transmission electron microscopy and EDX, recrystallization texture wasstudied by EBSD. Through the analysis of recrystallization texture evolution rule andsecond particle precipitation rule, the effects of second-phase on texture of SFG IFsteel were discussed. The main conclusions are as follow:1. When the annealing temperature is850℃, annealing time is from5min to20min, the ferrite grains gradually grow up, and plastic strain ratio r and strainhardening index n value gradually reduce with the annealing time increased. Whenannealing time is from20min to40min, ferrite grains fully grow up, r, n value reachmaximum. With the increasing of annealing time, the ferrite grain size and r、n valuechange little.2. During the annealing process, a lot of NbC, Nb(C、N) were precipitated fromfine grain high-strength IF steel. When the holding time is short, the second phaseparticle precipitated in steel is NbC, which is primarily circular and ellipticaldistribution. With annealing time increasing, the second particle in steel is Nb (C、N),which is primarily square distribution. The size of second particle in steel is increasedwith annealing time. The phenomen of PFZ near the ferrite grain boundaries wasobserved in the IF steels under different annealing processes and the width of PFZ isincreased with annealing time.3. The recrystallization texture of steel sheet has been greatly effected byannealing process. When the annealing temperature is850℃, with the increasing ofannealing time, strength of γ fiber in1/4layer annealing sample is obviously higherthan1/2layer, and the strength of α fiber in1/4layer annealing sample is less than1/2layer. The " Low CSL(Σ≤29) grain boundaries” in40min is significant higher thanother annealing time. When the annealing temperature is850, annealing time is40min,the characteristic of grain boundaries is better. With annealing temperature rises, thestrength of γ fiber increased at first, then present the trend of decline.4. The effects of second-phase on texture of SFG IF steel mainly depend on second phase particle size, shape and distribution. Small intensive second phaseparticle can make the dislocation distribution become more uniform, pinning force ofgrain boundary migration is big. So it has a negative impact on recrystallizationtexture. While pinning force of grain boundary migration which dispersiondistribution is small,{111} surface texture can be more fully development. So it isbenefit to drawing performance of SFG IF steel.According to this research, the proper processing parameter of the fine grainhigh-strength IF steel is that annealing temperature is850℃, annealing time is40min.The mechanical properties can be obtained by processing parameter as mentionabove: σs:225~290MPa, σb:410~460MPa, elongation:30~35%, Plastic strain ratio:1.8~2.2, strain hardening index:0.21~0.24.